In general, a fixing roller used in an electrophotographic image forming apparatus (e.g. a copying machine, a multifunctional apparatus, and a printer), particularly a fixing roller used in a color printer, includes a structure in which an elastic layer made of silicone rubber is formed around a metal core and a resin layer (surface layer) made of a PFA tube is provided around the elastic layer.
The fixing roller coated with the PFA tube is produced in such a manner that a PFA tube whose internal surface has been subjected to an etching treatment and to application of primer is fixed to the inside of a cylindrical mold, a core is inserted into the PFA tube, and then silicone rubber is poured into a space between the core and the PFA tube, and the silicone rubber is heated and cured, so that the PFA tube and the silicone rubber are molded integrally.
Further, in the fixing roller, a PFA tube with a thickness of 30 μm is generally used in order that elasticity of silicone rubber is impaired as little as possible, a wider nip width and a higher detachability of a sheet are secured, and unevenness in luster does not appear.
[Patent Document 1]
Japanese Unexamined Patent Publication No. 2001-312169 (Tokukai 2001-312169; published on Nov. 9, 2001)
However, the above fixing roller has inconvenience. When the fixing roller is used in a high-speed printer whose process speed (peripheral velocity of the fixing roller) is 300 mm/s or more, mechanical and thermal stress applied on the PFA tube is large, resulting in occurrence of wrinkles on the PFA tube.
An effective way to prevent such wrinkles is to make the PFA tube thicker (e.g. 40 μm or more), thereby increasing mechanical strength of the PFA tube. However, as a result of a test, the inventors of the present invention found that as the PFA tube becomes thicker, the PFA tube (resin layer) is more likely to be detached from the silicone rubber (elastic layer) at their interface. The reason is as follows: making the PFA tube thicker reduces flexibility of the PFA tube, and accordingly the PFA tube cannot sufficiently follow deformation of the silicone rubber, resulting in occurrence of shearing stress between the PFA tube and the silicone rubber.
In particular, in a case where the thickness of the silicone rubber (the thickness of the elastic layer) is large and/or hardness of the silicone rubber is low, deformation (shearing stress) of the elastic layer made of the silicone rubber gets large, which exhibits the above problem.
In order to solve the above problem, paragraph [0020] of Patent Document 1 discloses a method for adhereing a PFA tube, including steps (1) to (4) as follows: (1) a rubber roller made of silicone rubber is coated with a PFA tube, (2) an adhesive made of silicone rubber with self-adhesiveness is poured between the rubber roller and the PFA tube, (3) the PFA tube is squeezed by an O-ring so that the poured adhesive spreads from one end of the PFA tube to the other end of the PFA tube, and (4) the adhesive is heated and cured so that the PFA tube is adhered to the rubber roller. However, this method requires a far more complex manufacture process compared with a conventional manufacture method and has a difficulty in evenly forming an adhesive layer with a predetermined thickness, resulting in problems in quality and cost up.
For that reason, there is a request for a fixing roller that is different from the fixing roller disclosed in Patent Document 1 and that can prevent the above problem (the problem such that the elastic layer made of silicone rubber is likely to be detached from the resin layer made of the PFA tube).